The advent of acoustic metamaterials has opened up an emerging frontier in the control of sound transmission. A key limitation, however, is that an acoustic metamaterial based on a single local resonator in the unit cell produces a restricted narrow-band attenuation peak. When multiple local resonators are used, the attenuation peaks that arise—while numerous—are each still narrow and separated by passbands. Here, we present an acoustic metamaterial concept that yields a subwavelength sound transmission loss through two antiresonances—in a single band gap—that are fully coupled and, hence, provide a broadband attenuation range; this is in addition to delivering a high isolation intensity for both peaks that exceeds 100 dB within the 3–5 kHz range or 60 dB around 1 kHz. The underlying coupled-resonance mechanism is triggered by ensuring that two local resonances appear between two coincident frequencies formed by the intersection of the incident acoustic waves sound line with Bragg dispersion curves governing in-plane wave motion orthogonal to the direction of transmission. This phenomenon is nominally realized in the form of a thin single-panel single-material pillared-plate structure with internal contiguous holes, a practical configuration that lends itself to design adjustments and optimization for a frequency range of interest, down to subkilohertz, and to mass fabrication.

A simple and universal design principle to realize the generalized Brewster effect (GBE) is proposed. Based on the generalized Kerker effect (GKE), we can realize the GBE for arbitrary polarizations, any incident angles, and any frequencies on the traditional dielectric interface by the destructive interference of various of multipoles. An adjustable metallic meta-surface has been designed to achieve this effect experimentally in the microwave band. By adjusting the suspension height of meta-surfaces above the interface, the incident angles and frequency corresponding to the GBE can be modulated easily. For the two different polarized incident waves, the GBE can be realized at the same frequency and same incident angle, which shows the validity of the GKE. This design principle opens the door for realizing tunable GBE devices that can be applied in many scenarios.

We generalize similarity theory based on the scaling and solution invariance of the Boltzmann equation, coupled with the Poisson equation, and demonstrate similarity laws for radio-frequency (rf) discharge plasmas across three nonlinear transitional regimes, namely, the alpha-gamma mode transition, the stochastic-Ohmic-heating mode transition, and the bounce-resonance-heating mode transition. Fundamental plasma parameters, e.g., the electron power absorption, under similar discharge conditions are examined via fully kinetic particle-in-cell simulations, and electron-kinetic invariance is exemplified in similar rf discharge plasmas. The results unambiguously confirm the applicability of similarity laws for rf plasmas in extended operating regimes, and strengthen the foundations and framework of similarity physics with universality.

We propose a scheme for the detection of microwave-induced photons through a current-biased Josephson junction, from the point of view of the statistical decision theory. Our analysis is based on the numerical study of the zero-voltage lifetime distribution in response to a periodic train of pulses, that mimics the absorption of photons. The statistical properties of the detection are retrieved comparing the thermally induced transitions with the distribution of the switchings to the finite voltage state due to the joint action of thermal noise and of the incident pulses. The capability to discriminate the photon arrival can be quantified through the Kumar-Caroll index, which is a good indicator of the signal-to-noise ratio. The index can be exploited to identify the system parameters best suited for the detection of weak microwave photons.

Nitrogen-vacancy color centers in diamond have attracted broad attention as quantum sensors for both static and dynamic magnetic, electrical, strain, and thermal fields and are particularly attractive for quantum sensing under pressure in diamond anvil cells. Optically based nuclear magnetic resonance may be possible at pressures greater than a few gigapascals and offers an attractive alternative to conventional Faraday-induction-based detection. Here, we present ac sensing results and demonstrate synchronized readout up to 6 GPa but find that the sensitivity is reduced due to inhomogeneities of the microwave field and pressure within the sample space. These experiments enable the possibility of all-optical high resolution magnetic resonance of nanoliter sample volumes at high pressures.

While operating a quantum random-number generator (QRNG), it is extremely useful to have a model of the physical entropy source to guarantee that the device is delivering randomness of genuine quantum origin. In this work we consider a QRNG based on a gain-switched laser diode and we develop a model to quantify its phase noise. This model is based on the laser rate equations and the state-of-the-art techniques for the characterization of laser diodes used in lightwave systems. These tools let us achieve a faithful modeling of the phase noise and we verify its accuracy through comparisons with experimental measurements. Furthermore, the model can be used to select optimal parameters to maximize the QRNG performance and monitor the device behavior to detect malfunctioning or malicious tampering of the device.

Spintronic terahertz (THz) emitters based on ferromagnetic (FM)-nonmagnetic (NM) heterostructures are the focus of many experimental endeavors for THz sources. However, existing spintronic THz sources control emission by changing the external magnetic field, which is inconvenient for compact integration. Therefore, various modulated mechanisms are in high demand. Here, we present an electric-field-controlled spintronic THz emitter based on the ferroelectric (FE)-FM/NM structure that can emit THz radiation with voltage-modulated amplitude and polarization angle. The modulation depth of the THz amplitude is up to 69%, and the maximum rotation of the THz polarization angle is 28°. In addition, after removing the electric field, two different THz-emission states can remain unchanged, demonstrating nonvolatile behavior. Modulation is attributed to magnetoelectric coupling between the FM and FE layers, which modifies magnetization of the FM layer by the electric-field-induced strain in the FE layer, and thus, modulates THz emission. Furthermore, the possibility of a programmable array-type THz source with varied bias voltage is theoretically demonstrated, allowing a spatially modulated THz light to be generated directly without the use of additional spatial light modulators. We believe that this electric-field-controlled THz emitter has the potential to be used in next-generation on-chip THz sources, THz memory devices, and THz ghost imaging.

Recently, it was shown that dissipatively coupled laser arrays simulate the classical $XY$ model. We show that phase locking of laser arrays can give rise to the spontaneous formation of vortex and antivortex phase patterns that are analogous to topological defects of the $XY$ model. These patterns are stable although their formation is less likely in comparison to the ground-state lasing mode. In addition, we show that small ratios of photon to gain lifetime destabilize vortex and antivortex phase patterns. These findings are useful for studying topological effects in optics as well as for designing laser array devices.

Orientation-dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof-beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first-principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof-beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community.

We measure and analyze the light emission from a room-temperature n-type unipolar-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{Al}\mathrm{As}$ double-barrier resonant-tunneling diode (RTD) that occurs just above the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ band edge and peaks around 1631 nm. The emission is attributed to electron-hole recombination emission made possible by holes generated in the high-field region on the collector side of the device by interband tunneling and impact ionization, which contribute comparable hole densities, according to our analysis. Although the external quantum efficiency (EQE) in our experimental configuration is rather low (≈2 × ${10}^{-5}$ at 3.0-V bias), limited by suboptimal output coupling, the internal quantum efficiency (IQE) is much higher (≈6% at 3.0-V bias), as derived from the experimental EQE and a radiometric analysis. To check this value and better understand the transport physics, we also carry out an independent estimate of the IQE using a combined interband-tunneling impact-ionization transport model, which yields an IQE of 10% at 3.0-V bias. The satisfactory agreement of theory with experimental data suggests that a RTD designed for better hole transport and superior optical coupling could become a useful light-emitting device, while retaining the intrinsic functionality of high-speed negative differential resistance, and all without the need for resistive p-type doping.

The interaction of light and electrons has been one of the important frontiers for generating ultrashort electron pulses in free-electron lasers, ultrafast science, and dynamical analysis of matter. However, the generation of a relativistic attosecond electron beam remains a challenge. In a photocathode radio-frequency (rf) gun, this work identifies a regime for obtaining an isolated or a train of relativistic attosecond electron pulses. A photoelectron beam is generated from the cathode under the illumination of a driving laser pulse in the electron gun, which is subsequently microbunched to optical wavelength scale by the field of a synchronized radially polarized laser (RPL) pulse that is focused near the cathode surface. The rf field in the gun cavity simultaneously accelerates the electron pulse to several MeV. The rf field causes a velocity differential inside the electron beam as it passes into the region near the focal point of the RPL (microbunching is processed), and the microbunch at the head is substantially faster than that at the tail. Using this knowledge, the spacing between consecutive microbunches, referred to as bunch spacing, may be controlled across a wide range by regulating the velocity difference, which can be accomplished by tuning the phase and amplitude of the rf field. Numerical simulations show that a train of attosecond bunches with tunable bunch spacing can be generated using realistic laser parameters corresponding to current GW-power-level laser systems, and an isolated attosecond pulse can be obtained when the driving laser pulse is about 50 fs. This regime may result in opportunities in ultrafast electron diffraction and microscopy, free-electron lasers, and other applications that require high-energy electrons with the temporal structure of single-cycle light.

Flexible oxide thin films with uniaxial magnetocrystalline anisotropy $(K_u)$ not only have many applications in flexible electronics, but also provide an ideal low-crystal-symmetry material for a fundamental understanding of the competitive interplay among magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy. However, an understanding of the mechanical tuning of magnetic parameters in flexible oxide films with $K_u$ has not been realized up to now. In this work, epitaxial flexible hexaferrite ${\mathrm{Ba}\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ thin films with a room temperature out-of-plane $K_u$ of 1.1 × ${10}^5$ J/m${}^3$ are fabricated. The continuous and controllable tuning of magnetic parameters is found to be realizable by different bending radii of the film. The changes in magnetic parameters are symmetrical under tensile and compressive bending due to the dominance of film geometry and bending-stress directions, which are perpendicular to the easy axis of $K_u$. The micromagnetic simulations further show that the magnitude of $K_u$ plays a critical role in the mechanical tuning performance of the film, where high $K_u$ drastically reduces the role of stress anisotropy and increases the magnetic bending repeatability of the film by suppressing bending-defect-related anisotropy degradation. This work not only provides an understanding of the role of the direction and magnitude of $K_u$ in the flexible thin films, but also demonstrates that the flexible hexaferrite film is one of the best materials for mechanical tunable devices up to the millimeter-wave region.

In the coming era of precision medicine, there is a high demand for the development of approaches for in vivo tracking of biomolecules and measurements of their local environments. In this work, we present an approach for the simultaneous imaging of magnetic nanoparticle (MNP) concentration, temperature, and viscosity. The proposed approach allows one to determine the spatial distribution of the MNPs, while the MNPs are used as nanosensors for simultaneous measurements of the spatial distributions of temperature and viscosity. This technique has great potential for providing not only tools for disease diagnostics and therapy, but also insights into further understanding of the biophysical mechanisms involved in tumor tissues during cancer development and tumor metastasis.

In this report, a rectifying surface acoustic wave (SAW) device is proposed and simulated based on a simple inclined phononic crystal (PnC) heterojunction, consisting of monolithic pillars on $\mathrm{Si}$ substrate. The designed nonreciprocal operation principle is initially based on the frequency alignment of the surface-coupled guiding bands in the first half of the $\mathrm{Pn}\mathrm{C}$ with the local-surface-resonance (LSR) band gap in the second half of the $\mathrm{Pn}\mathrm{C}$, along two different equivalent incident directions. Benefiting from flexible LSR band-gap engineering, we tune the band-gap central frequency by optimizing the structural dimensions of the pillars in a small chip area, which is not achievable in conventional Bragg band gaps without varying the lattice constant. The other physical principle that dominantly affects the broken reciprocity in our proposed structure is the induced SAW shear-to-sagittal mode conversion at a limited frequency range in the trapezoidal $\mathrm{Pn}\mathrm{C}$ half, which depends on the incident direction with respect to the inclined cut line of the $\mathrm{Pn}\mathrm{C}$. Moreover, we optimize the spacing gap between the PnCs to modulate the elastic coupling strength between the half $\mathrm{Pn}\mathrm{Cs}$, and prove a significant SAW nonreciprocity of 34 dB at a frequency of 6.9 GHz, in addition to an acceptable rectified transmission of about −10.68 dB, by the proposed $\mathrm{Pn}\mathrm{C}$-based operation principles. The presented design benefits from a simple $\mathrm{Si}$-based structure and a CMOS-compatible fabrication process, without the need for any external excitation, and it is a promising miniature and efficient SAW rectifying candidate for wireless-communication applications.

We develop a homogenization theory for a spatiotemporally modulated wire medium. We first solve for the modal waves that are supported by this composite medium, and we show peculiar properties such as extraordinary waves that propagate at frequencies below the cutoff frequency of the corresponding stationary medium. We explain how these unique solutions give rise to an extreme Fresnel drag that exists already with weak and slow spatiotemporal modulation. Next we derive the effective material permittivity that corresponds to each of the first few supported modes, and write the average fields and Poynting vector. Nonlocality, nonreciprocity, and anisotropy due to the spatiotemporal modulation are three inherent properties of this medium, and are clearly seen in the effective material parameters. Lastly, we validate that homogenization and spatiotemporal variation are not necessarily interchangeable operations. Indeed, in certain parameter regimes the homogenization should be performed directly on the spatiotemporally modulated composite medium, rather than the stationary medium being homogenized first and then the effect of the space-time modulation being introduced phenomenologically.

Propagation of easy-plane magnetic precession can enable more efficient spin transport than conventional spin waves. Such easy-plane spin transport is typically understood in terms of a hydrodynamic model, partially analogous to superfluids. Here, using micromagnetic simulations, we examine easy-plane spin transport in magnetic strips as the motion of a train of domain walls rather than as a hydrodynamic flow. We observe that the motion transitions from diffusive to fluidlike as the density of domain walls is increased. This transition is most evident in notched nanostrips, where the domain walls are pinned by the notch defect in the diffusive regime but propagate essentially unimpeded in the fluidlike regime. Our findings suggest that spin transport via easy-plane precession, robust against defects, is achievable in strips based on realistic metallic ferromagnets and hence amenable to practical device applications.

Current-induced spin polarization in magnets has recently attracted much attention because out-of-plane spin polarization is allowed, as opposed to polycrystalline nonmagnetic heavy metals. We investigate the $\mathrm{Pt}$-impurity-doping dependence of current-induced out-of-plane spin polarization in $\mathrm{Fe}$ thin films with the use of magnetocircular dichroism. Current-induced out-of-plane spin polarization is clearly observed and attributed to anomalous spin-orbit torque, in which the spin-orbit torque induced by the spin Hall effect in $\mathrm{Fe}$ acts on the magnetic moments of $\mathrm{Fe}$ at the surfaces, generating out-of-plane spin polarization. Notably, current-induced out-of-plane spin polarization is significantly enhanced with increasing $\mathrm{Pt}$ concentration. $\mathrm{Pt}$ impurities with a large spin-orbit interaction induce the extrinsic spin Hall effect of $\mathrm{Fe}$, and consequently, the anomalous spin-orbit torques are enhanced. Our results provide a guiding principle for strengthening current-induced spin polarization in magnets.

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating, and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\sim {10}^{-4}{\lambda }_0^3$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operational principle of such photonic gap antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or the polymer CYTOP as the gap material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of approximately $1000$ for air gaps and approximately $400$ for CYTOP gaps over a spectral bandwidth of $\mathrm{\Delta }\lambda \approx 300\mathrm{nm}$ at $\lambda =1.25\mu \mathrm{m}$. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of approximately $3000$ for air gaps and approximately $1200$ for CYTOP gaps.